Recently, a crank assembly adapted for a speed-changing bicycle includes a crank arm and at least two chainrings secured with the crank arm. One of the chainrings may be connected to a rear freewheel which is connected to a hub of a rear wheel via a chain. Thus, when the rider pedals to drive one of the chainrings being rotated clockwise, the chain movably drives the rear freewheel rotating clockwise such the rear wheel rotatably drives the bicycle moving forward.
The conventional crank assembly is formed by the crank arm secured with the chainrings via a spider. However, the spider of the conventional crank assembly may cause a limitation about selecting the chainrings and the corresponding crank arm. For example, owing to the structure limitation of the spider, the crank arm and the chainrings of the conventional crank assembly adapted for the mountain bike are limited to be only suited to being provided with the mountain bike, that is to say, the crank arm and the chainrings of the conventional crank assembly adapted for the mountain bike cannot be replaceable for the road bike. Simultaneously, the chainrings have to keep some space for being secured with the spider such that the chainring may not be enough securing space for another element used to be stably secured with another chainring.
On the other hand, reducing the weight of the bicycle is a goal in the bicycle industry. In order to reduce the weight of the crank assembly of the bicycle, the two chainrings are integrated to each other. Furthering, the crank arm is directly secured with one of the chainrings such that the crank assembly do not have to be assembled with the spider, as resulting in reducing the weight of the crank assembly of the bicycle. However, when one of the two integrated chainrings of the crank assembly has a breakdown and need to be displaced a new chainring, the two integrated chainrings must be simultaneously displaced with two new chainrings such that the users disburse more cost.
Therefore, it is necessary to improve the above problems of the conventional chainring assembly.